This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast that is controlled by a microprocessor.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Because the frequency of the inverter is much higher than 50-60 hz., the inductors for an electronic ballast are much smaller than the inductor in a magnetic ballast.
Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit that increases the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt input) to 300 volts or more, for operating a lamp and for providing power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one). The boost circuit, the inverter, or both, can be self-oscillating, triggered, or driven.
There are many additional circuits described in the prior art for automatically shutting off a ballast in the event of a fault. A fault may be a problem with the ballast itself, e.g. high temperature, or with the lamp, e.g. a broken filament. These faults are addressed in a number of ways, all of which increase the complexity and cost of an electronic ballast. One cannot add circuits ad infinitum to accommodate every conceivable fault because the circuitry becomes too complex, the fault detectors interact unfavorably, and the ballast simply becomes too expensive to be commercially viable. U.S. Pat. No. 5,493,181 (Shackle et al.) discloses a ballast that accommodates several faults while efficiently providing power for a gas discharge lamp.
As known in the art, the number and type of gas discharge lamps continues to increase, creating the need for a like number of ballasts to meet the requirements of each lamp type and to meet the inconsistent safety regulations among the several markets for gas discharge lamps. This proliferation of ballast types is compounded by the various power line voltages around the world, e.g. 120V, 127V, 220V, 230V, or 277V. There remains a need for an electronic ballast capable of efficiently supplying the correct voltage and correct current for several lamp types while meeting the various voltage and safety requirements of global markets.
When there is a fault and an electronic ballast shuts down, particularly an electronic ballast that can accommodate several different faults, it is not known what happened. The only clue that something is wrong is that a lamp is off when it should be on. For someone about to remove a lamp or inspect a ballast, it would be desirable to have some idea of what went wrong.
There are several microprocessors on the market that include memory, input/output (I/O) ports, timing, and analog to digital conversion capability. These microprocessors, also known as microcontrollers, are used in a wide variety of applications, including the control of an electronic ballast. U.S. Pat. No. 5,680,015 (Bernitz et al.) discloses a ballast for a high intensity discharge lamp wherein a microprocessor controls a driven half-bridge inverter having a series resonant, direct coupled output. The Bernitz et al. ballast is described in more detail in the Detailed Description section.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast that is adaptable to many types of gas discharge lamps with minimal changes in hardware.
A further object of the invention is to provide an electronic ballast that is adaptable to differing safety requirements of world markets with minimal changes in hardware.
Another object of the invention is to provide an electronic ballast that can accommodate a wide range of faults without damage to the ballast and, if a lamp should fail, without further damage to a lamp coupled to the ballast.
A further object of the invention is to provide an electronic ballast that can communicate the nature of a fault in response to an inquiry.
Another object of the invention is to provide an electronic ballast that can operate on any common power line voltage around the world.